1. Field of the Invention
The present invention relates to an apparatus and method for providing protection against an electrocution hazard in an appliance arising from a miswiring condition in an electrical source that is to electrically supply the appliance. In particular, the present invention provides such protection where the putative ground line of the electrical supply is improperly electrically charged or is not properly connected.
2. Description of the Related Art
Modern single-phase, alternating current (AC) electrical wiring employs a three wire or line configuration. A first wire is an ungrounded conductor that is commonly referred to as the “hot” wire. A second wire is a grounded conductor that is commonly referred to as the “neutral” wire. The third wire is a grounding conductor that is commonly referred to as the “ground” wire. The ground wire is typically a bare wire, whereas the hot and neutral wires are sheathed in color-coded insulation, the hot wire insulation typically being black and the neutral wire insulation typically being white.
Despite these conventions, miswirings occur with alarming frequency, especially where the wiring has been installed by an inexperienced person, such as a homeowner. A miswiring may occur at an electrical outlet, at the electrical panel to which the electrical outlet is connected, or somewhere between the electrical outlet and the electrical panel, for example, at a switch or at a connection or work box. The miswiring may consist of the swapping of two or all three of the wires, a failure to connect a wire properly, or the omission of a wire.
Simple testing devices which plug into an electrical outlet can be used to determine if an electrical outlet is correctly wired. An example of such a tester is described in U.S. Pat. No. 3,967,195 to Averitt et al. issued Jun. 29, 1976, which is incorporated herein by reference. These testing devices typically employ the various on-off lighting combinations of a set of three lights to indicate whether the electrical outlet is correctly wired or if wires have been switched or are unconnected. However, the same inexperience or incorrect installation work that causes an electrical outlet to be incorrectly wired in the first place may also result in the failure to test the electrical outlet for correctness. Even when an electrical outlet is tested for correct wiring upon installation, later changes to the wiring in the electrical panel or in work boxes between the electrical outlet and the electrical panel can result in the electrical outlet being effectively miswired.
The most common miswiring condition is the swapping of the two insulated wires, that is, the hot and neutral wires. Due to the nature of single-phase AC power, many lamps and appliances plugged into an electrical outlet in which the hot and neutral wires have been swapped will operate, nonetheless. However, if the lamp or appliance is not double-insulated or is not grounded, for example, by a grounding wire of the lamp or appliance being electrically connected to the ground wire terminal of the electrical outlet, the lamp or appliance could be an electrocution hazard for a person who comes into simultaneous contact with the lamp or appliance and a true electrical ground. The availability of ground fault interrupter devices, as are well known in the art, has reduced the frequency of injuries resulting from this problem. However, the use of ground fault interrupter devices on electrical outlets is far from universal and the ground fault interrupter devices themselves must be periodically checked to determine if they are in working order.
A much less common, but potentially lethal, electrical miswiring problem is the swapping of the hot wire and the ground wire. This results in the ground blade receptacle of the electrical outlet being electrically hot. Appliances and lamps plugged into an electrical outlet in which the hot and ground wires have been swapped will not operate. Although this inoperativeness signals that a problem exists, it does not identify what the problem is. The user may initially misdiagnose the problem as being with the appliance or electrical panel or an intermediate switch rather than with the electrical outlet. However, the user may be electrocuted before he discovers the true cause of the problem or even the fact that a problem exists. The grounding wire on the appliance or lamp side of the electrical outlet is typically attached to the portions of the appliance or lamp with which the user can be expected come into contact, which for simplicity of expression are referred to herein as the housing of the appliance or lamp. A grounding wire properly connected to the housing is intended to shunt any voltage leaking to the housing to an electrical ground through the ground wire. When the ground wire of the electrical outlet is correctly wired, this arrangement prevents a dangerous voltage difference from existing between the housing and electrical ground voltage and so protects a user from being electrocuted who simultaneously comes in contact with the housing and an electrical ground.
However, where the hot and ground wires have been swapped in an electrical outlet, the ground wire becomes electrically energized or live with the AC voltage the hot wire was intended to have, for example, 120 VAC. This causes the ground wire of an appliance or lamp plugged into such an electrical outlet to conduct this high voltage potential to the housing of the appliance or lamp. Anyone who comes into simultaneous contact with the electrified housing of the appliance or lamp and an electrical ground may be electrocuted. This hazard is particularly acute where the appliance or lamp is one that is intended for use in or near water, for example, water pumps, drain cleaners, and certain work lamps.
Moreover, it is also possible for an electrical outlet to have an electrically energized ground wire connected to its ground blade receptacle, but have no connections to the blade receptacles that are normally connected to the hot and neutral wires. In such a case, none of the lights of a common three-light tester will light, thus suggesting that either the hot wire is not connected in the electrical outlet or that there is no power to the electrical outlet, for example, because the power has been disconnected by a breaker, fuse or switch or that the electrical outlet has not yet been wired. Nonetheless, unless the person making the test takes immediate action to correct the problem or to post some kind of warning against use of the electrical outlet, a second person having no knowledge of the test may attempt to use the electrical outlet with deadly results.
The omission of a ground wire connection to an electrical outlet also presents an electrocution hazard. For instance, where the ground receptacle of an electrical outlet is not electrically connected to a ground line, any electrical short between the hot wire and the ground receptacle may result in the ground receptacle becoming electrically energized. Although a fuse or circuit breaker usually protects against electrical shorts, there is no protection in this case because the absence of a ground line connection prevents there from being any current flow due to the electrical short until a person or some other conductor completes the circuit to ground and then it may be too late. Such an electrical short may occur if an uninsulated portion of the hot wire comes into contact with either a portion of the ground receptacle or, where the electrical box containing the electrical outlet is metal, any portion of the electrical box. This might happen, for example, if an insulating twist on wire connector on the hot wire gets dislodged when the wires are pushed back into the electrical box during the installation of the electrical outlet or if too much insulation was stripped away from the end of the hot wire to make a connection leaving a portion of the hot wire exposed. Where the electrical box is metal, it too may become electrically energized under these circumstances.
Additionally, where there is no shorting of the hot wire to the neutral receptacle, the omission of a ground wire connection to the electrical outlet is a particularly insidious problem because the person who plugs an appliance or lamp having a three-prong plug into an electrical outlet having a three-prong receptacle expects that the electrical outlet will electrically ground the appliance or lamp. Where the ground wire in the electrical outlet is missing or unconnected, the sense of security the user gets is not only false, but is potentially lethal.
What is lacking in the art, therefore, is a device which protects against the electrocution hazard caused by an electrical outlet having an electrically energized ground blade receptacle, while permitting the appliance or lamp to be electrically connected to the ground line of the electrical outlet into which it is plugged when the electrical outlet is properly wired. Ground fault interrupter devices do not provide protection against this condition because they operate by looking for an imbalance in the current flows between the hot and neutral wires and do not monitor the ground wire. U.S. Pat. No. 5,754,114 to Gnadt issued May 19, 1998, U.S. Pat. No. 5,606,480, to Nevo, issued Feb. 5, 1997, and U.S. Pat. No. 4,843,514, to Bodkin issued Jun. 27, 1989, all of which are incorporated herein by reference, disclose devices that break the connections in the hot and neutral lines to the appliance or lamp when a miswiring condition is detected. However, such devices do not protect against the housing of the appliance or lamp becoming an electrocution hazard when the appliance or lamp is connected to an electrical outlet having an electrically energized ground blade receptacle. U.S. Pat. No. 6,023,400 to Nevo, issued Feb. 8, 2000, which is incorporated herein by reference, discloses a device that employs a virtual ground. Although this device provides protection against an electrical outlet having an electrically hot ground wire receptacle, it does so at the cost of totally eliminating any electrical connection between the ground line of the electrical outlet and that of the appliance or lamp, even when the electrical outlet is properly wired.
Thus, there is a need for a device that provides protection against the electrocution hazard that is caused by an electrical outlet having an electrically energized ground blade receptacle, or more generally, by a miswired electrical source in which the putative ground line is electrically energized.